text:
How to adjust time of useful consciousness for pilots acclimatised to high altitudes?
The time of useful consciousness (TUC) is the length of time an average pilot, breathing ambient-pressure air without supplemental oxygen, is capable of functioning usefully at a given altitude.  For instance, at FL180 (approximately 18 kilofeet AMSL), the TUC is 20-30 minutes for a steady ascent to this pressure altitude, or 10-15 minutes following a rapid or explosive decompression; at FL280 (~28 kft AMSL), TUC is 3-5 minutes for a gradual ascent (and half as long for a rapid/explosive decompression), and, at FL400 (~40 kft), TUC is only 15 to 20 seconds, even for a steady ascent (and a mere 7 to 10 seconds following a sudden decompression).1
However, healthy humans are capable of a very considerable degree of acclimatisation to high pressure altitudes (and resultant low partial pressures of oxygen); humans have demonstrated the ability to acclimatise to, and live and function indefinitely at, altitudes up to 19.5 kft, and long-term acclimatisation is thought to be possible up to at least 24-26 kft (although complete acclimatisation takes an increasingly long time - up to weeks to months in the upper reaches of this range - at very high altitudes).2  As extreme examples, a pilot living in La Rinconada (elevation 16.7 kft AMSL) could have an indefinite TUC as high up as FL180 (where, as mentioned above, the average pilot would be able to function without supplemental oxygen for at most half an hour), and someone just back from climbing Mount Everest sans oxygène supplémentaire could potentially be able to maintain an indefinite TUC up to at least FL220 or so (where an average pilot would have a TUC not exceeding ten minutes or so).
A high-altitude-acclimatised pilot would obviously have considerably longer TUCs for a given altitude than a sea-level pilot (even at altitudes where even a highly-acclimatised pilot cannot function indefinitely); how does a pilot acclimatised to high altitudes adjust the times in the standard TUC table to account for this?

1: Despite the 50% reduction in TUC following a rapid or explosive decompression, these, paradoxically, tend to be less dangerous (except in the rare cases where the decompression is sufficiently violent, or occurs from a sufficiently inconvenient part of the aircraft, to cause severe damage to aircraft structure and/or critical systems, or to physically suck one or more of the occupants from the aircraft), due to their violently obvious nature, than gradual decompressions (where the aircraft occupants can easily become incapacitated by hypoxia before they even realise that they have a pressurisation problem).
2: Beyond 24-26 kft AMSL (in what is aptly known as the Death Zone), long-term acclimatisation is mostly thought to be beyond the human body's capabilities; however, some degree of incomplete acclimatisation (allowing one to survive and usefully function at these altitudes without supplemental oxygen for considerably - sometimes very considerably - longer than an unacclimatised person would be capable of, although not indefinitely) is possible (which is how it's possible for a sufficiently-acclimatised person to climb Mount Everest without using any supplemental oxygen at all), and some scientists (from Bolivia, appropriately enough) dispute that there is an upper limit at all (or, at least, one below 29 kft AMSL).

Given the TUC at some altitudes is seconds, acclimatization is going to give you negligible increase in conciousness

The simple answer is "they don't". Training will dictate that at altitude, they need to be able to don masks within a few seconds, regardless of an individuals tolerance to altitude. The problem isn't that you have X seconds of full-cognition and then you pass-out, your brain function degrades during that period. It's best to get your mask on and oxygen flowing before doing anything else, and this is how pilots train.
Euphoria and a false sense of ability is a side-effect of altitude sickness/hypoxia. There are numerous videos out there showing altitude chamber tests where individuals are given simple tasks as the oxygen saturation decreases. These tasks can be as simple as those infant "put the shape in the hole" boxes, and it is surprising to see how well the people think they are doing versus how poorly they are actually performing. These two things combine for a very dangerous situation and you, as a pilot, need to recognize that this can happen and to mitigate that as quickly as possible, without trying to take into account your own tolerance (which can change day-to-day) for altitude.
Check this one out, this is a military pilot getting cards wrong, this is a stark demonstration of the effects. These symptoms happen in seconds. Watch how fast his cognition comes back when given oxygen.
Or this one which demonstrates euphoria and the inability to follow directions or perform basic actions.

I can second Ron's answer (you don't) and add a personal anecdote; back in the mid 80s I was invited to join some skydivers taking a High Altitude Indoctrination Course at the Defense and Civil Institute of Environmental Medicine in Toronto when they had an open spot.  A jumper had to have taken the course to be able to jump from altitudes above 12000 feet in Canada.  I was no longer jumping but a chum invited me along to fill in a seat opened by a cancellation.
The chamber ride was only to 25000 because the intent was only to experience hypoxia symptoms, not decompression events, and especially to learn what your unique hypoxia-onset symtoms were.
We all sat in the chamber (a yellow steel box with military transport style seats - I think there were about 6 or 8 of us) with oxy masks on, each with clip board/paper with vertical rows or columns of boxes next to geometric shapes.  After the air was slowly pumped out until we were at 25000 pressure altitude, we were told to disconnect the mask hose at the seat base and start drawing matching shapes of each shape in the box.  When we felt symptoms coming on, were to observe them to the extent we could, then reconnect the oxy mask hose before being incapacitated if possible (there was a guy in the chamber to do it for you if you were).
My observations, albeit a distant memory:

When I disconnected the oxygen source in the thin air, I couldn't tell the difference at all, sitting stationary.  The air doesn't feel thin. You would have to be exerting yourself to become short of breath.
When I was drawing my shapes, by about the 2rd column, a minute or two in, I could tell that my shapes were starting to get mis-shapen, but I didn't think it was a big deal.  I remember thinking my shapes were fine although they were lopsided.
My chum was a couple seats down and was a smoker.  He went into giggling mode in maybe 1 to 2 minutes, laughing like he was on Nitrous Oxide.  The smoking cut his tolerance time down a LOT.
My first obvious symptom was tunnel vision (a good thing - if your first symptom is euphoria like my friend had, that's bad).  I remember my peripheral vision vanishing (it's impossible to describe) and only being able to see the clipboard, my knees and my hands.
I think I then started to enter euphoria stage, but for me the sensation came on really quickly, like a shot of alcohol right to the brain.  If you've ever had Nitrous at the dentist, it is very similar.  It came on so fast it startled me, and that plus the tunnel vision prompted me to reconnect the hose (you're already primed for action by the briefing and it can be enough to get you to act even though you are mentally compromised, which was the point of the training). If it had come on gradually, I probably would have done nothing and enjoyed the high until I passed out.
My smoker friend had to have his hose reconnected for him as he started to fade (everybody noticed the giggling stopping), quite a bit sooner than the non-smokers.
When I reconnected the hose, the tunnel vision and drunk sensation vanished immediately after the first or second breath.  It was like going from dunk to instantly un-drunk in a heartbeat. Very strange experience in itself.
I looked at my shapes; first column nice, second column, slightly off, third column like a House of Mirrors of distorted shapes.

While certainly interesting, I fail to see how this answers the question asked by the OP.

Yes I did.  "You don't" is basically the answer.   In any case, do we have to enforce the posting rules to this level of anal retentiveness?


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Why do the A320’s primary flight controls have hydromechanical backups only for the rudder?
The A320 is primarily a fly-by-wire aircraft, with the pilots’ joystick and rudder-pedal inputs not going directly to the flight-control surfaces, but, rather, to the aircraft’s flight-control computers, which then decide what, if any, control-surface deflection is necessary, and send the requisite commands to the hydraulic actuators which move the control surfaces:1

However, in the event of a complete failure of the aircraft’s flight-control-computer system, the A320 can switch to a backup mode known as mechanical law, which bypasses the FCCs completely.2  In this mode, the only one of the primary flight controls to remain active is the rudder, using an emergency cable mechanism which sends the pilots’ control inputs directly to the rudder actuators; with the elevators, ailerons, and spoilerons all inactive, pitch control is maintained using manual horizontal-stabiliser trim (which uses a similar cable mechanism to send trim commands to the hydraulic motors  powering the stabiliser jackscrew) and varying engine power, and the ability to control roll separately from yaw is lost completely (to roll the aircraft in one direction or the other, it has to be yawed in the desired direction using rudder inputs and differential engine thrust; the resulting sideslip then causes the aircraft to roll towards the trailing wing, due to the positive slip-roll coupling generated by the A320’s rearward wing sweep):

The reason the elevators, ailerons, and spoilerons are unuseable in mechanical law is that, unlike the rudder, they have no direct hydromechanical connections to the cockpit controls at all, being connected only via the flight-control computers (thus leaving them cut off from the pilots’ control inputs in the event of a total failure of those computers).  If the A320 did have hydromechanical backup controls for the elevators, ailerons, and spoilerons, the pilots would be able to control the aircraft in each of its three axes separately (rather than with roll dependent on yaw, which provides very sloppy roll control with a significant time delay between the initiation of the yaw and the resulting sideslip-induced roll, and with a small but significant initial rudder-induced rolling moment in the wrong direction), and would be able to do so using the same types of control inputs as would be used in normal, fly-by-wire flight (albeit likely with considerably-increased control forces and a markedly-different force gradient), rather than having to switch to a completely different input style in mechanical law:

So why is the rudder the only one of the A320’s primary flight controls to have hydromechanical backups allowing its use in mechanical law?

1: The A320’s flight-control system, and that of all later Airbus aircraft, has several different fly-by-wire flight-control modes.  In some of these (direct law, alternate 1/2A/2B law), the pilots’ flight-control inputs in some or all control axes are simply passed straight through the computers unfiltered, with the computers parroting whatever the pilots ask of the aircraft without any of the translation present in the usual operational mode (known as normal law); however, the pilots’ inputs are still passed to the computers first, and only then to the control-surface actuators, rather than going directly aft.
2: The pitch and roll/yaw axes can each also revert to mechanical law separately in the event of a major fault in one of the primary flight controls (for instance, a double elevator jam would cause the aircraft to revert to mechanical pitch control), while still leaving the other in fly-by-wire mode.

Failures beyond direct law doesn't really worth investing. The usefulness of mechanical law is debatable, in the sense that if it's needed at all and how useful it is when it's needed. The use of FBW is an indication that the plane is uncontrollable beyond direct law. Instead of spending money and weight on mechanical law, you are much better off adding another layer of redundancy to FBW.

I think you may have answered your own question, Sean. I don't really know much about this stuff, but while looking at the graphs, I can understand a little bit. The only question is: are these images from credible sources (e.g., faa, airbus, or an a320 operator)?

@user3528438, there was one failure which lead to use of the mechanical backup fairly recently, on Feb 28th 2018. It is the only one I've ever heard of.

The thing is your first picture is not really correct. The links from side-sticks go through the “magic” box, but the links from trim wheels and pedals don't. Instead there is a link from the “magic” box to the pedals and trim wheel. So if the “magic” stops working, there is no change in configuration. The pedals and trim wheels just continue working because they are connected differently.
The side-stick output needs to go through the “magic” box; that's the point of using it. So this way they have a mechanical backup in case all the five components of the “magic” box fail without actually having any extra links. Any other mechanical backup would need extra links, which would negate the advantages of the design for something that so far only seems to have happened once.

"The side-stick output needs to go through the “magic” box; that's the point of using it."  Why would that be the case for the joystick, but not the rudder?

@Sean, I suppose it would make some sense to make the rudder pedals fully electronic as well, but they were easier to connect mechanically and just adding the pilot command with the yaw damper command is closer to the desired outcome (which is probably selecting slip-skid ball target) than it would be for the pitch and roll commands.

Right, but, if the yaw commands can do just fine bypassing the magic box, then why can't the pitch and roll commands?

@Sean, because the pitch and roll commands are requests for vertical acceleration and roll-rate respectively, which requires varying the surface deflection according to the actual aircraft response that depends on speed, weight and balance, and because the meaning of pitch and roll commands varies with the aircraft attitude and angle of attack. So it takes logic and a feedback loop to even interpret the commands.

The mechanical backup is only meant to maintain basic controllability during a temporary failure of the whole fly-by-wire system. As the OP mentioned, the rudder is deemed sufficient for basic roll control, while the horizontal stabilizer is deemed sufficient for basic pitch control even without any stability augmentation. That being said, the mechanical backup is not meant to achieve continued safe flight and landing (I would be really surprised if the mechanical backup meets all of the ARAC minimum controllability guidance); it's expected that at least one flight control computer can be rebooted to take over control after the initial failure (Ref. Airbus A320 Crew Manual).
The reason for relegating the mechanical backup to rudder and trimmable stabilizer, instead of to elevator and ailerons, pretty much has to do with the choice of the passive sidestick as the pitch and roll inceptor. It lacks mechanical linkages to translate its motion to external assemblies, nor does it have a feel unit to provide feedback for the airloads. This leads to significant weight savings and simplicity in design, and is the primary advantage of the passive sidestick.
I suspect that the reason for even having a mechanical backup scheme is due to mistrust in FBW systems for civil aviation when they first came to use. As trust in redundancy and dissimilarity grew, newer generations of Airbus aircraft (e.g. A380, A350, A220) did away with it completely, and instead have an electrical backup servo control loop on each of the axis.

I know this sounds a bit far fetched, but what if there was an EMP bomb or something similar that went off?

@Ipydawa Then it's game over. The FADEC is all electronic so you will lose engine control, along with your flight controls, perhaps also IMUs, ADS, etc, depending on how much electrical components are within the standby instrumentation.

@JZYL, the stand-by instruments are a display too these days, and IIRC display simply the data from ADIRU 3.

@Ipydawa An A320 will likely crash if hit by an EMP. It will also crash if hit by an anti-aircraft missile. It's a civilian aircraft; not designed to fly into a war zone.

@GavinS.Yancey That said, I’m now mildly curious what the A400M does (since that is a military aircraft).

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Why can’t a single-fan-beam radar determine the elevation angles of its primary targets?
A while back, I asked about the reason for one of the well-known limitations of most ATC primary radars (their inability to determine target altitudes).1  The answers boiled down to “single-beam fan-beam2 primary radars can determine the azimuth of targets (from what direction the radar beam was pointing when the return popped up) and the slant distance to targets (from the amount of time it took for the beam to travel from the radar to the target and back), but they have no way of knowing the elevation angle of a target, since they can’t know what part of the beam bounced off the target”.34
But hang on - why can’t the radar determine what part of the beam hit the target?  An aircraft reflecting a radar beam behaves (at first approximation, anyway) as a point source (or, at least, a very-small-angle-coverage source), and, while finding the direction of point sources emitting in the radio spectrum is harder than finding the direction of shorter-wavelength point sources (such as those in the infrared, visible, or gamma-ray spectra), it is by no means impossible, and the technology for doing so (involving a spinning directional antenna, and known, descriptively, as ADF - Automatic Direction-Finding) has been mature for a very long time - otherwise, one of the mainstays of air navigation in remote or poorly-developed areas, the NDB, would be utterly useless!
It should not be very hard, at least conceptually, to use a similar system (essentially an ADF receiver turned 90 degrees onto one side [so as to scan vertically, rather than horizontally] and resized for PSR6 rather than NDB frequencies) to determine the elevation angles of primary targets, allowing the altitudes of these targets to be calculated, so why isn’t this generally done in practice?

1: In contrast, military air-defence radars generally do display meaningful altitude information for primary targets (the better to intercept boogers with), as do a few other specialised types of radar (such as precision approach radar, which is designed to provide altitude and azimuth information about an incoming aircraft to an air traffic controller to allow the execution of an accurate ground-controlled approach down to low minima).
2: Radars that illuminate an entire vertical slice of sky at once.
3: In contrast, pencil-beam radars (which illuminate only a small spot of sky at any one time) and dual-beam radars (which have two radars, mounted one on top of the other, and use trigonometry and the time-of-flight difference between the two radars to calculate the exact location of a target in space) are able to determine the elevation angles of targets, but come with their own drawbacks: pencil-beam radars take far longer to scan the whole sky than fan-beam radars do, while dual-beam radars require twice as much radar per radar (and radar equipment is expensive).
4: In addition to denying ATC altitude information about primary targets, this also introduces a considerable amount of uncertainty into the horizontal position (and, thus the ground track) of such a target, forcing the use of greater separation distances between aircraft than would be necessary in an all-secondary-target5 environment.
5: Secondary surveillance radar (SSR), unlike primary radar, is not a true radar system; instead of bouncing radio waves off an aircraft’s skin, it sends an interrogation signal to the aircraft’s transponder, which then replies with information about the aircraft’s altitude (among many other things).  Newer transponders also provide the aircraft’s horizontal location; for aircraft with older transponders, secondary radar falls back to determining the target’s azimuth and slant distance using antenna-orientation and signal-time-of-flight information, and adds the transponder-provided altitude to this to calculate the aircraft’s exact position in three-dimensional space.  Secondary radar provides far more information than primary radar, and provides more reliable target tracking than primary radar (especially for aircraft at low heights above terrain, which would otherwise be hidden by ground clutter, and for small, nonmetallic, and\or stealth aircraft, which reflect less primary-radar beam than big metal non-stealth aircraft), but only works with targets with an operating transponder (whereas primary radar can detect sufficiently-reflective targets with nonfunctional, or no, transponders, and even non-aircraft targets, such as weather, flocks of birds, or parts detached from aircraft).
6: Primary Surveillance Radar.

Because it's not often needed for ATC application. It's often used by CIWS though https://en.m.wikipedia.org/wiki/Kashtan_CIWS . Note the "90 degree" antenna between the guns.

Because the ATC system gets aircraft altitude information from the aircraft's altimeter.

I expect it's because the accuracy of the altitude calculation would decline with slant range to an unacceptable level within the useful range of the PSR.  So much easier to just encode the pressure altitude passively at the aircraft itself, providing an accuracy of +/- 50 ft, that is consistent at all ranges, and broadcast that.

Did you really mean "boogers"?

I've often wondered whether the radar at an air force base 2.5 miles away from an (authorized) model airplane field can detect a model airplane similar to this ( https://www.horizonhobby.com/product/airplanes/airplanes-14501--1/bind-n-fly/conscendo-advanced-1-5m-bnf-basic-pkz8150 ) at all, and if so, whether it can determine whether it is closer to 200 feet or  2000 feet AGL... at a location near the center of the country... I doubt they have an air defense radar system but who knows... maybe grounds for a new ASE question...

What you propose is called a monopulse radar. Monopulse antannas have two feeds and beams, slighly to the left and right of boresight or above and below. In transmit mode, both feeds are connected in parallel to create centered beam and a single pulse is transmitted this way. In receive mode, the phase or amplitude difference in the two channels is used to find the exact direction. Many civilian primary and secondary radars use the monopulse technique with a left and right channel. Many military radars also have an upper an lower channel for height finding.
However, height finding is simply too inaccurate for civilian air traffic purposes. It happens that all airplanes operate at pretty much the same altitude and that barometric altimeters are extremely accurate, which led to uncomfortably low vertical separation minima, often as low as 500'/150m. There is no way to achieve such accuracy with radio direction finding from 50km away, which is why they don't even try.

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Where is KBOS runway 15R's localiser array?
Of the eight major-runway directions at KBOS, five (4R, 15R, 22L, 27, and 33L) are equipped with ILS (ILS-III for 4R and 33L, ILS-I for the rest).  Four of these (4R, 22L, 27, and 33L) have a localiser array some way past the end of the runway (or, in 4R's case, on the other side of Belle Isle Inlet in line with the runway).
15R, however, has no visually-apparent localiser array (this image, and all others in this question, were made by me screenshotting Google Maps), although the glideslope antenna for 33L (15R going the other way) can be seen:

Contrast, for example, 22L, where the localiser is immediately obvious:

Now, the EMAS bed just past the end of 15R does have a bunch of posts sticking up from it (visible mostly via the shadows they cast on the light-grey EMAS blocks); however, these are almost certainly approach lights, given the suspicious similarity of their arrangement to that of the approach lights on the pier beyond:

So where is 15R's localiser?


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Why does elevator dropdown occur in some loss-of-flight-controls accidents, but not others?
In some instances of an aircraft suffering a catastrophic failure of its primary flight controls, the loss of control force on the elevator allows said elevator to drop down under its own weight, causing an immediate crash.
Examples:

THY 981 (DC-10, 1974, Paris)
LOT 007 (Il-62, 1980, Warsaw)

In other loss-of-flight-controls accidents, however, the elevator either stays stuck at its position immediately prior to the control failure, or floats in the airstream:

JAL 123 (B747, 1985, Mt. Takamagahara)
LOT 5055 (Il-62, 1987, Warsaw)
UAL 232 (DC-10, 1989, Iowa)
DHL OO-DLL (A300, 2003, Baghdad)

Why does elevator dropdown occur during some, and only some, loss-of-flight-controls accidents?

@ymb1, the article says horizontal stabilizer, but it clearly means the elevator, because the foreplane dropping with gravity would cause pitch up and also because it is then mentioned that using manual trim could have worked (but there wasn't enough time to recover).

An elevator wouldn't 'drop down', even if all mechanical links to it were snapped. First, aerodynamic forces are much greater than its weight. Second, control surfaces, especially on aircraft with [hydro]mechanical linkage (such as IL-62), are normally weight balanced. That said, the behaviour of the airplane with relaxed vs. stuck control will be different, but neither necessarily leads to immediate pitch down.

The mechanical or hydromechanical control system transmits the forces both ways, potentially amplified in the later case. When the pilot trims the plane so he is not applying any force on the control column, all the forces on the elevator are balanced and this includes gravity.
So when the control links are broken, the elevator will return to its trimmed position. In that condition, the plane will maintain speed with pitch, oscillating around the equilibrium in a Phugoid oscillation.
In the later four cases, the planes were likely trimmed, and had engine power left, so they remained controllable for some time.
In the LOT 007 case the aircraft was almost certainly not trimmed – the pilot could just hold the control column and not bother with the trim – and it lost most power, so it pitched down immediately.
The THY 981 either for some reason was not trimmed, or maybe the stabilizer moved a bit as the control links severed. I don't think there is enough information to tell for sure.
Also in the LOT 5055 case it is not clear the plane did not initially pitch down as the trim was still operating (until the spreading fire burned the power supply to it) and they had enough altitude to regain control by using it.

Even if the elevator starts out in a trimmed position, it won't stay there once the hydraulics/cables/servotabs/whatnot holding it in trim are suddenly no longer functioning.

@Sean, that's what I am saying: “elevator will return to its trimmed position”. The point is that that position will not be flying straight, so the aircraft will pitch. Up, or down, depending on the position of the trim.


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Weather sensors required for ILS approach operations?
Was flying into Sullivan County this weekend and had to run the RNAV instead of the ILS. ATC advised there was a NOTAM for the weather sensors (which we had already confirmed.) Does an ILS approach require that the airport have functioning weather or is this just a case of a 2-for-1?

Weather sensors were out so you couldn't get winds & ceiling?

@CrossRoads yeah, was advised not to trust information on the ATIS frequency.

Whether or not an ILS may be conducted without proper weather reporting will depend on the regulations under which the flight is operated.
In simple terms, a flight operated under 14 CFR 91 may perform an ILS without this weather reporting.
This may also be allowed under the eligible-on-demand rules of 14 CFR 135, if all pertinent requirements are met.
Otherwise, a flight operated under 14 CFR 135 or 14 CFR 121 will generally not be approved to conduct an ILS without approved weather reporting.
This answer applies under FAA jurisdiction.


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What is the ground clearance area required for certification for a blimp/airship that needs to be tethered?
Does anyone know how much ground space needs to be allocated for keeping a blimp/airship tethered? I assume it is likely a circular area with the diameter being (2 x total length) + safety factor.

Enough for it to swing around I presume

Are you asking for regulations or recommendations? Is there a specific country you're interested in?

The aircraft will most likely be operating in Europe but built in the U.S., from my understanding U.S. regulations typically translate almost everywhere. I'd love any insight on certification based on United States standards regarding this issue.

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Can an aircraft get certified for different runway lengths through the FAA based on the amount of payload it is carrying during said takeoff?
When certifying a new aircraft through the FAA, will they certify the AC for just one takeoff distance based on the max payload capacity, or can you get the AC certified for multiple runway lengths based on the amount of weight it is carrying at the time of takeoff?

Generally an aircraft will be certified for a range of runway distances based on headwind/air temp/weight, a correction chart will be provided in the POH allowing you to compute the runway distance for various conditions. For example here is the chart for the PA-28-161 I trained on

The big planes have similar charts, you can find the 737-800 numbers here and if you want to go to the really heavy metal the 747 info can be found here.
The AC that covers aircraft certification can be found here and interestingly the distances are computed from standstill to the point at which the lowest part of the aircraft is 35ft above the ground.

(1) The takeoff distance on a dry runway is the greater of the two
  distances depicted in (a) or (b) below (see page 63 of above linked doc). The distances indicated below
  are measured horizontally from the main landing gears at initial brake
  release to that same point on the airplane when the lowest part of the
  departing airplane is 35 ft. above the surface of the runway.

Interestingly the FAR does not seem to address the weight at which the takeoff distance tests are done.

Aircraft are not certified for one particular runway distance. They are certified to an absolute max structural takeoff weight, for airborne performance or engine out obstacle clearance.
Performance charts will dictate the runway length required for any given takeoff weight and given weather conditions. If the runway is too short for that weight then the weight may be reduced by fuel load or payload. In that case the max weight is “runway limited”.
So in effect, Yes, an aircraft may use any runway length provided the weight allows sufficient obstacle clearance performance.

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What are different types of weights of an aircraft?
When looking at different aircraft's specifications or in POHs/AFMs, there are several different weights of the aircraft mentioned (e.g. maximum take-off weight, maximum landing weight, operating empty weight).
What do they mean and how they are different?

The different terms represent different weights or masses1. For each flight, the weights are taken into account for several reasons.
A brief description about these is below:
Manufacturer's empty weight (MEW)
Also called Manufacturer's Weight Empty (MWE) or Licensed Empty Weight
It is the weight of the aircraft "as built" and includes the weight of the structure, power plant, furnishings, installations, systems and other equipment that are considered an integral part of an aircraft.
This excludes any baggage, passengers, or usable fuel2.
Zero-fuel weight (ZFW)
This is the total weight of the airplane and all its contents (including unusable fuel), but excluding the total weight of the usable fuel on board.
As a flight progresses and fuel is consumed, the total weight of the airplane reduces, but the ZFW remains constant.
Maximum zero fuel weight (MZFW) is the maximum weight allowed before usable fuel and other specified usable agents (engine injection fluid, and other consumable propulsion agents) are loaded.
Operating empty weight (OEW) (Roughly equivalent to basic empty weight on light aircraft)
It is the basic weight of an aircraft including the crew, all fluids necessary for operation such as engine oil, engine coolant, water, unusable fuel and all operator items and equipment required for flight but excluding usable fuel and the payload.
Payload
It is the carrying capacity of an aircraft. It includes cargo, people, extra fuel. In the case of a commercial airliner, it may refer only to revenue-generating cargo or paying passengers.
Maximum takeoff weight (MTOW)
This is the maximum weight at which the pilot of the aircraft is allowed to attempt to take off3.
Regulated takeoff weight (RTOW)
Depending on different factors (e.g. flap setting, altitude, air temperature, length of runway), RTOW or maximum permissible takeoff weight varies for each takeoff. It can never be higher than MTOW. More information is on this answer.
Maximum landing weight (MLW)
This maximum weight at which an aircraft is permitted to land3.
The following image depicts takeoff weight components.

Image Source
Maximum ramp weight (MRW)
also called maximum taxi weight (MTW)
It is the maximum weight authorized for maneuvering (taxiing or towing) an aircraft on the ground.
Aircraft gross weight
It is the total aircraft weight at any moment during the flight or ground operation. This decreases during flight due to fuel and oil consumption.

1: As mentioned (below in comments by SentryRaven), several recent references use the term mass instead of weight. Having a Physics background, I tend to agree and know the difference between weight and mass. I did not use mass as most of the references I included referred mass as weight. However, using correct terminology, all the weights mean mass. Thanks SentryRaven for pointing it out.
2: What is the different between usable fuel and unusable fuel?
Usable fuel is the fuel on board an aircraft that can actually be used by its engines. The small amount of fuel that cannot be drained from the tanks is the unusable fuel. For calculation of range, usable fuel is used. For weight and balance total fuel (usable + unusable) is used.
3: This restriction is due to structural, design or operational limitations.

AFAIK, they are no longer called weights but masses. The PPL training and examination material no longer uses MTOW but only MTOM: maximum take-off mass. The skybrary also refers to MTOM. Maybe leave a little remark in your otherwise very nice answer?

Also: +1 for image attribution and going Q&A style. ;)

Also related (and alluded to above) are the two most common "load" figures: Useful Load (the maximum gross takeoff or operating weight minus the operating empty weight), and Payload (what the aircraft can carry in cargo/passengers/personnel when loaded with the fuel required to make the planned trip).

@SentryRaven Since the U.S. is still (foolishly in my opinion) not using the metric system as our standard, I doubt that we will anytime soon use M instead of W since pounds are a unit of force, not mass. Also, I think the SKYbrary entry could be better written. As it is now, it would likely be interpreted as saying the aircraft's mass in pounds does not vary by altitude when, in fact, it does ever so slightly. The change, of course, is insignificant for aircraft weight purposes.

I believe MLW should refer to footnote #3, so I changed that. Please revert if I'm incorrect

@Terry, As far as I can tell, "pound" is a unit of mass (defined as 1 lb = 0.45359237 kg)  and there is a "force pound", abbreviated lbf, that is a unit of force defined as 1 lbf = 1 lb × g (g is gravity of Earth), allowing people to be sloppy with their dimensions.

@JanHudec We may be looking at a generational thing. As I remember both my high school and college physics (1950s and 1960s), we were taught that in the English (Imperial?) system that the unit of mass was the "slug", the unit of force the "pound". The equivalent metric units were the Kilogram and the Newton. There was no mention of a "force pound."

@Terry, I think it's more a domain thing. "Engineering" system has pound for both mass and weight and live with g coming up in Newton's second law, "Gravitational" system uses pound for force and slug = lbf·s²/ft for mass and "Absolute" system uses pound for mass and poundal (pdl) = lb·ft/s² for force. The first is most common, but impractical for physics, which is why the other two are used there.

@Terry, also note that the metric system actually has similar set of variations with units like kilopond = kg·g and even hyl = kilopond·s²/m, but these systems have fallen into disuse and SI became almost universal except for few uses of CGS, which is just scaled by powers of 10, and various natural units in particle physics and cosmology. While in imperial both mass-pound and slug are widely used.

Read this from left to right, from detailed to generalized. Boxes on the right sum up the separate items which they run across.

The maximum landing weight is what the manufacturer has certified for landings. Generally, this is MTOW minus some fuel.
Note that every manufacturer has his own standard and might draw the lines between the different weight groups differently. Say, for one the engine quadrant is part of the propulsion system, whereas the other sees it as part of the control system.

Lets see if I can make it more simplified.
The aircraft has:

Basic Empty Weight (BEW) is the weight of the aircraft "as built" and includes the weight of the structure, power plant, furnishings, installations, systems and other equipment that are considered an integral part of an aircraft before additional operator items are added for operation.
Dry Operating Wieght (DOW) BEW + Weight of Crew (Pilot + Cabin including their bags) + Pantry
Operating Weight (OW) DOW + Takeoff fuel (i.e. Ramp Fuel - Taxi fuel)
Maximum Zero Fuel Weight (MZFW) DOW + Payload (anything put on the aircraft that generates revenue to the company, e.g. passenger, baggage, cargo, mail and fret)
Maximum Taxi Weight (MTW) MZFW + Ramp fuel
Maximum Takeoff Weight (MTOW) MZFW + Takeoff fuel, or MTW - Taxi fuel
Maximum Landing Weight (MLW) MTOW - Trip Fuel

Max landing weight is in reality mzfw plus some fuel in the wings which will not cause structural failure upon touchdown.
For example an aircraft carrying max fuel for transatlantic flight must dump fuel when returning for landing due to an emergency,this procedure will ensure that the wings of that airplane won't break upon touchdown .
Otherwise good answers over all.

Wrong. Your ZFW can be well below MZFW but you still have a Max Landing Weight to contend with — more fuel in the wings than being at MZFW in that case.  The two limits are separate limits that have to be observed, and are only somewhat related.

In military aviation both a Normal Take Off Weight and a Maximum Overload Take Off Weight are sometimes quoted, for the C130J these figures are 155,000 lb and 175,000 lb respectively.


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Supersonic aircraft desire features
It is possible to modify a supersonic aircraft such as it can fly under the radar? If so, what are the three most important aspects to consider to be able to do this effectively?

When flying at low level, under the radar waves from a ground installation, the air is very dense. So, if you want to fly fast, you need 1) low drag, and 2) high thrust. The third most important aspect is probably a flight control system that enables the aircraft to do this without possibly hitting the terrain, by not relying on only the pilot skills to fly safely. I nominate drag as the most important aspect, because it's better to minimise that than have to counter act with increased thrust. At supersonic speed, low drag is significantly achieved by swept wings that fall within the shock cone formed by the nose of the aircraft. The F-111 is an old design, but could fly supersonic at low level, had variable swing wings, and featured terrain following radar. So, it can/has been done. But..if the aircraft is already designed, the second and third aspects can be modified, but drag cannot be changed.


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At what age does a regularly maintained airframe most commonly start needing frequent repairs?
I am looking to buy a Cessna 182.  I am trying to work out the sweet spot between age and price.  I am looking for a plane that will be really reliable.  I am looking to spend several months flying around the USA with my wife and prefer not to spend my whole time dealing with mechanics.  Some questions:

At what age does a regularly maintained airframe most commonly start needing frequent  repairs?
At what age does the depreciation slow down (so I can buy and sell the plane after three years with minimal loss)?

I think this question might be considered too broad/opinion based and impossible to answer. But basically you have airframe, avionics and engine to consider. I would consider a well maintained 182 with 5000 hours or less on airframe to be no problem (everyone else will surely have a different answer). For the engine, you want to make sure it is regularly flown. I rather have an engine with 1200 that was overhauled 2 years ago, then an engine with 120 hours that was overhauled 20 years ago. Finally, for avionics, find a plane that has what you want already installed.

David, I have removed the 2nd question because it would still call for opinions. I have left the other two because I think they might be answerable. (I am not sure though, we'll see what the community thinks)

Never, depending on your definitions of "well maintained" and "frequent"; 2) After 15-20 years or so, I think you will find that well-maintained planes appreciate (at least before inflation) due to supply & demand.

OK thanks for the help.  This is my first time out :)

I'm tempted to say "5 minutes after you get the key for any aircraft". There's always something to tinker with after all and that's about the time it takes to run to the aircraft and start tinkering...


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Why do helicopters typically have blades at the top?
This question has several great answers showing that it's perfectly possible for a helicopter to fly with its blades underneath the rest of the aircraft.
There are some obvious issues with having a helicopter's blades at the bottom, to do with proximity to the ground and being in the way when someone wants to get in or out of the helicopter. Is the answer that simple, or are there other reasons related to flight dynamics, manufacture, or something else?

Yes the answer is that simple, that mincer needs to be out of the way so it doesn't chop peoples' head or feet off, or slashes into a bit of concrete and shatters into 100 pieces scattered around like bullets. That is the main reason why the light 2-seater R22 has the rotor mounted on a pylon, at 2.7 m it is out of the way of most people.
Image source
Flight dynamics wise, a rotor underneath the helicopter has speed stability in the hover, which makes the helicopter easier to control. The rotor acting as a pusher makes it more efficient; being closer to the ground means more lift in ground effect and softer take-off and landing. Manufacturing:

The rotorhead of an underslung rotor would look pretty similar to the top mounted one, a hinge offset head would line up the fuselage with the rotor disk, top or bottom.
The rotor would have to be situated away from the fuselage to allow for the upwards coning and tilting under flight conditions - for instance mounted on an upside down pylon quite similar to the R22.

So all in all an underslung rotor poses no particularly vexing engineering problems and provides improved stability and better performance - it would just be an immensely impractical machine.

It does have one engineering problem I can think of - how would the landing gear pass through the rotor? Unless it had massive insect-like legs that reached past the rotor tips.

@Skyler well I was going to lump that under impracticality. They could stick 2 wheels out of the rotor mast, and a tail wheel from the tail rotor.

@Skyler The blades would need a really fast shaft passer.

There were some experimental "helicopters' with rotors on bottom:

A current design, also capable of transitioning to horizontal mode
http://aliptera.com/#ADR-1

That's not a helicopter. (But it's a cool machine).


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Is the A320 family allowed to take off with the flaps retracted?
Is it legal for the A320 family (A318, A319, A320 and A321) to take off without flaps?
An answer to this question states that the A300 can take off without flaps. On the other hand, the 737 family of aircraft is not approved for a no-flap takeoff. (Under the right circumstances, you might survive one, but the margins for losing an engine would be unacceptable, so no data is published for such an operation.)

The fact that some aircraft can (noted in the linked answer) doesn't mean he A-318/319/320/321 family necessarily can.  Plus, the fact that you might SURVIVE an inadvertant no-flap takeoff doesn't mean it would be legal... while your 2-engine performance might be adequate, engine-out performance might be unacceptable. The linked Q/A never addresses the A-319 family at all... not a duplicate.

The 737 isn't approved for a no-flap takeoff. You can survive one under certain conditions if everything goes right, but AFAIK there is no published data or procedures for doing so. I don't have an answer for the A319 etc, but the logic to close this as a dupe presumes what I suspect to be the wrong answer as correct.

The original question referred to "A319 and similar aircraft", hence my earlier comment, but if the question has been changed specifically to refer to the A320 family, then that no longer applies. @RalphJ

the biggest problem I see for A320 is that maximum tyre speed is 195kts. In clean config, depending on weight and outside condittions, that might not be enough to fly


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Could an ultralight hypothetically takeoff without power on if it were windy enough?
I don't know if takeoff would be the right term, but the wing should generate lift with any headwind.

That works only if the aircraft is somehow prevented from getting blown away. The concept of kite-flying applies.

It could, but eventually drag would cause the ultralight to start moving in the same direction as the wind and it would lose any apparent forward airspeed.

And if you tow it to the ground so it doesn’t move too much with the wind you have a kite.

A wing will produce lift when exposed to a headwind. And yes, if the wind is strong enough, a properly tethered airplane, ultralight or not, will 'become unstuck'.

Even a tethered A380 could become "unstuck" with the right wind. The OP didn't mention tethering though.

The ultralight could takeoff with a strong enough headwind, but would be blown away in the process. This would not only be..ahem...very dangerous, but would cause the ultralight to lose a lot of airspeed. If it were tethered to the ground, it would fly like a kite. The wind would pick it up, and the strings or rope would prevent it from losing control or flying away.


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If a circling airplane (AC-130) fires a bullet how does the plane's trajectory alter the shell's flight path?
Background: An AC-130 is a military aircraft that has a cannon that fires out the side of its fuselage.
If an AC-130 (or similar aircraft) is traveling in straight, level flight at a constant rate of speed calculating gun lead is straightforward.  For example if the plane is traveling forward at 100 meters per second, the target is 1000 meters to the side and its cannon shell travels at 1000 meters per second simply fire 1 second or 100 meters early to score a hit.
However what if the AC-130 is traveling in a circular orbit at 100 meters per second around its target instead of beside the target?  I'm not sure how to calculate gun lead in that circumstance.
Hopefully the image I attached below will help this make sense.  Thanks so much in advance for any input!

This is a purely math-based question, but the solution is straight forward. Given any two extremely close time instance, the plane can be considered travelling along a straight line in that timeframe (for more details, see circular motion topics in Physics). Therefore by simply taking a snapshot of the position and orientation of the plane at the instant the bullet is fired, the solution is the same as it is in the straight line case.

I don’t believe they do compute a lead as there is no relative translation between the aircraft and the pylon.

The scenario is almost the same as in your first diagram. In the millisecond or two that the shell takes to travel the length of the barrel the plane turns only a very slight amount. Once the shell leaves the barrel it’s all linear physics and the continuing turn of the plane has no effect on the shell. There might be a few centimeters difference at the target.

As others have commented the instantaneous motion of the aircraft at time of firing is the same as linear motion. One thing that does make a slight difference however is the outward G-force acting on the shell as the aircraft turns. That would act to reduce the muzzel velocity very slightly which will change the landing point a smidge but likely not enough to bother about.

@Trevor_G  No... there's no 'centrifugal force effect' at all, since that 'force' disappears the moment the shell is shot.

@xxavier: Once it leaves the barrel, yes.But inside the barrel, Trevor is right in that the shell experiences the same centrifugal force as everything else onboard the plane - transferred via the barrel casing.

@Scrontch The circular motion of the barrel will communicate its peripheral speed to the shell, but not any radial component to be added to the shell's own 'radial speed'.

@xxavier: I'll argue like follows:
Put a marble inside a straight narrow tube. Put the tube radially on a centrifuge. The marble will travel to the outside. According to your logic the tube only transfers a perpendicular force on the marble at all times and thus couldn't make it move radially. Yet the marble moves radially along the tube because of the centrifugal force. It is this radially outward movement in the barrel that the gunpowder must now additionally compensate for in the OP question. So we get a (slightly) reduced muzzle velocity.

@Scrontch -- wouldn't your comment not apply, in a case where the turn was "coordinated", meaning that the slip-skid ball was centered?

@quietflyer I'm sorry, i don't understand the terms slip-skid or coordinated here. My comment applies in any case i would say. My argument is that the bullet in the barrel isn't in free motion (yet). It is forced (by the barrel casing) upon a partly circular orbit (when you would plot the path of the bullet in the barrel it would be part of a spiral actually), that means it experiences at least part of the centrifugal force and thus its radial acceleration is reduced and muzzle velocity is lower than in the linear case.

The bullet leaves the barrel with the combined velocity of barrel muzzle velocity plus the instantaneous aircraft true air speed velocity (actually, the instantaneous true air speed velocity of the tip of the muzzle). That puts the velocity vector of the bullet ahead of the line of the barrel (ahead of the 90 degree wing line). The fact that the barrel is rotating in inertial space is irrelevant. Whether the aircraft is straight and level, turning towards the tank, or away from it, if, at the moment the bullet leaves the muzzle, it is traveling in inertial space towards the tank, (and sufficiently above it for gravity drop) it will hit it.
And since the bullet's velocity leaving the barrel is a simple vector sum of the muzzle velocity, and the aircraft's instantaneous velocity at that instant. The velocity vector of the aircraft before, (or after), the bullet leaves the barrel is of no relevance.
So if the aircraft is moving due North at 100m/s and the muzzle velocity is 1000m/s due East relative to the aircraft, the bullet is actually traveling
SQR(100^2 + 1000^2) = 1004.98 ~1005m/s in a direction slightly North of due east by an amount equal to the arctangent of 100/1000 or ArcTan(0.1) = 6.345 degrees
So the bullet is traveling 1004 m/s on a heading of 90-6.34 degrees = 83.66 degrees true.
So, to hit the tank, the pilot has to fly so that the gun barrel is pointed behind the tank by 6.34 degrees (and above it by a sufficient amount to account for gravity drop)

Though, to a certain degree, I wouldn't expect this to matter much in practice - the AC-130's main armament puts enough "ouch" downrange per round (or, for the GAU, "ouch" per second) that small drifts in accuracy, especially at the low altitudes it flies at, likely wouldn't have much of an effect.

@SebastianLenartowicz, all the “ouch” is still focused in at most a few meters radius and the correction is by tens of meters at least, so it definitely does matter. Of course the gunner does not have to think about it, because the sight does it for them.

Wouldn't the rotation of the aircraft put a torque on the bullet that might deflect it slightly in the direction of the rotation of the aircraft?

No, The bullet is spinning and this creates a stability and resistance to any applied torque. But more relevant, there are two ways in which motion occurs, or in which motion can be changed. The velocity of an object can be changed by a linear force, and the rotation rate of an object can be changed by a torque. Torque on an object does not change it's velocity. It only changes it's rate of rotation. And so even if the bullet was a non-spinning sphere, and a torque did change it's rotation rate, it's velocity, (and trajectory), would not change and it would still impact at the same point.

When a projectile is fired from a gun in a fixed wing aircraft or helicopter in a direction perpendicular to the direction of flight, the airflow across the muzzle has an effect on the trajectory which is known as "bullet jump", the deflection can be upwards or downwards depending on the direction of the rifling of the gun (normally right hand or clockwise) and the direction of the airflow. In the case of the AC130, in which the weapons are mounted on the port side the deflection would be in a downward direction. This occurs because the combination of the rotation of the projectile and the airflow causes a difference in air pressure between the upper and lower sides of the projectile. Technically this is known as the Magnus Effect.

This is more of a physics question than an aviation one. As other answers say, the effect on the trajectory of the bullet is determined by just the instantaneous velocity. However, if the purpose of taking into account the effect is to calculate lead time, then circular travel will be different from linear. If you decide you need to fire 1 second earlier, then with circular motion the velocity of the aircraft when you otherwise would have fired is different from the velocity 1 second earlier. So with a tight circle, you can't just start at the time you otherwise would fire, take the velocity at that point, and calculate how much earlier you need to fire based on that. If you want an exact solution, you'll have to set up a formula for the bullet's trajectory and solve numerically.
Also, when you watch the bullet, it will appear to curve from your frame of reference (it's really you who is turning, but it looks like the bullet's direction is changing), which can make it difficult to learn to fire from a circling vehicle.

Gunships adjust fire by seeing the impacts, not by watching the bullet trajectory - which can't be observed anyway, since they don't use tracer rounds. Also, everything is about computing an impact for pressing the trigger "right now" rather than at some defined time in the future.

@RalphJ OP expressed it in terms of firing early.

Good points here in this answer. Took almost 3 years for anyone to get to this! This is highly relevant to points raised in the body of the original question, if not in the actual title.

In the case of a gatling type weapon the detailed design of the gun and mounting and the rate of fire would all have to be taken into consideration as they would determine the direction and magnitude of the velocity vector imparted to the projectile by the rotation of the barrels while it was being loaded and fired, as well as the internal ballistics of the ammunition and the individual barrels. AC130s were formerly armed with 7.62mm, 20mm and 25mm gatling guns, but now as I understand it these have all been replaced with 40mm single barreled guns.

The heart of the question is whether or not it matters that the gunship airplane is travelling in a circular path.  No it does not.  All that matters is the aircraft's instantaneous linear velocity at the instant the bullet leaves the muzzle of the gun.
Some of the answers addressed the effect of the spinning barrel assembly of a Gatling-gun type weapon.  This doesn't really appear to be what the question was about, but the same principal applies: the instantaneous linear velocity of the barrel at the instant the bullet leaves the muzzle will affect the bullet's trajectory.  To know what direction the barrel is travelling at the instant the bullet leaves the gun, we have to know how long it takes the bullet or shell to pass through the barrel, and how quickly the barrel assembly is spinning.  If rate of spin of the barrel assembly is not constant, the effect of the barrel's motion on the bullet's trajectory will not be constant.
This Wikipedia page mentions a firing rate of 1,800 rounds per minute for one of the Gatling-gun weapons used on the AC-130U.  Since there are 5 barrels, the rate of spin of the barrel assembly must one fifth of this figure, or 360 revolutions per minute.  Assuming that each barrel assembly rotates around a circular path with a diameter of 8 inches, i.e. a radius of 4 inches (a wild guess), the instantaneous linear speed of each barrel due to this rotation is about 9043 inches per minute or 8.6 mph.  This appears to be a rather trivial number compared to the flight speed of the aircraft.  (To correct this estimate for a different known radius of spin, divide by 4 and multiply by actual radius of spin in inches.)
If we assume that the bullet is fired at either the top or the bottom of barrel's circular path, and we assume that the barrel only travels a short distance before the bullet exits the barrel, then the barrel would be travelling roughly horizontally when the bullet leaves the barrel.  This would mean that the instantaneous linear velocity of the barrel due to rotation of the barrel assembly should be either added to, or subtracted from, the instantaneous linear velocity of the aircraft, depending on whether the bullet is exiting the barrel at the "top" or the "bottom" of the spinning barrel assembly, and which direction the barrel assembly is spinning, and on which side of the aircraft the gun is mounted (the left side in the case of the AC-130U.)

2000 SPM divided by 5 barrels would give the barrel assembly rotating at 400 RPM, not 2,000. Otherwise, I agree with the central point of the answer.

@RalphJ -- good point, will edit

@RalphJ --also my intention was to use 6 inches as the diameter of rotation, not the radius.  A 6 inch radius might be too much and might give too high a linear velocity.  Have reworked using a 8 inch diameter / 4 inch radius.

That 8" number is the right ballpark. I'd have guessed a little wider than that, but it's been a while since I've been close enough to a gunship. For purposes of discussion, that's plenty accurate.


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What is the flight trajectory of a commercial airplane in regards to altitude v/s distance?
So, my friend and I got into an argument regarding the flight trajectory of a plane. He said that for half the journey the plane is ascending and for the rest, it is descending and hence making a parabolic curve. I argued that it ascends and then stays at the same altitude for the journey before descending to make a trapezoidal curve. After some contemplation, I think that both of us are right just that the baseline reference is different where he took it as a straight line between the two places without the curvature of the earth and hence it would make it parabolic while I took the trapezoidal curve by accounting the curvature of the earth.
We can also say it is from a different of view with him being in space while I am running on the ground to measure the altitude.
So, can someone clarify and tell who is right and whether what I think is correct or not?
It would be helpful if someone can actually plot the two graphs.
Thank you.

short helicopter flights can look vaguely parabolic/elliptical afaik fixed-wing aircraft trajectories never do

Both of you are somewhat right - in the way you mention.
Normally, flight profiles (i.e. the altitude flown over the distance covered) are depicted assuming (yes I know this might be controversial... oh dear ...) a flat earth. Constant altitude will then be represented by a straight horizontal line, so it is easy to see when the aircraft is in level flight and when it actually is climbing or descending:

Note, by the way, that some short flights will have virtually no level cruise phase:

Also, a flight with optimum efficiency (minimum fuel used to cover a given distance) will look roughly parabolic in this profile view. Real life flights aren’t operated that way though, mainly due to constraints on airspace use. Airspace constraints will also be the reason the descent phase in the second picture looks so stepped.
You could, as you say, instead of the profile view show a 2-dimensional view with the curvature of the earth shown and the flight path plotted above it. That would then look curved for most of the flight. I don’t see much use in this representation for normal aircraft, though, unless we start to cover orbital operations...
Pictures are screenshots of historical commercial flights found in Flightradar24 app.

Your friend is not correct and you are somewhat correct. For now I will consider long-haul flights as the effect is more visible. As the aircraft takes off it climbs towards it cruising altitude, say 33,000ft. Then during the cruise the airplane uses fuel meaning that the aircraft is effectively losing weight. Since during a steady cruise, lift = weight the aircraft will have an excess lift if nothing but the weight changes. This will make the aircraft climb to higher altitudes were the air is thinner and thus more lift is required.
Thus on some long-haul flights you can see that the flight ends at 38,000ft. Then the pilot initiates the decent for landing. Depending on ATC the aircraft is allowed to gradually climb during cruise to higher altitudes. If that is not allowed the aircraft will climb once every hour or so to a higher flight level.

"Depending on ATC the aircraft is allowed to gradually climb during cruise to higher altitudes."
Never seen an FIR or ATC which allows that. However, there have been a time where the Concorde could climb gradually over the North Atlantic while crossing, because basically it did a ballistic flight.

"This will make the aircraft climb to higher altitudes ...." Not really true in practice as the autopilot will hold the aircraft at the assigned altitude. The increased altitude capability will be available, though, for a climb to a higher altitude when the ATC clearance permits. While you can request a block altitude that would allow a continuous climb, that is not often done for an enroute cruise climb.

@Terry, thus if you turn off the autopilot the aircraft will gradually climb during cruise

@Brilsmurfffje  I think that's true, but I have enough uncertainty that I'll defer to those more technically qualified than I. The problem is, at least on the 747-100/200, as I remember, with the zfw c.g. near the preferred 26.6% mac, as you burn fuel the c.g. moves aft. Thus, with the autopilot off and no change in the trim or thrust, the nose will come up slowly. I don't know how much of the climb will be due to that vs the lightening of the aircraft. I only did that once over the Pacific when given a "climb when able" clearance, and I decided to play rather then do the usual step climb.

Each aircraft has an optimal service ceiling (or altitude so to speak), depending on type and usage (military vs passenger vs small piston plane).
As for passenger planes, airlines plan their routes the most optimal way: lateral and horizontally as well. It's a combination of aircraft weight, upper winds, shortest way, time vs distance etc.
When you leave the the lateral options away and focus on the vertical options airline pilots mostly want to fly the optimal altitude to save fuel or time (or a combination of those two). The optimal altitude increases for the duration of the flight: the lighter the plane, the higher is its optimal altitude - except if for some special wind situations.
Therefore a typical passenger plane climbs to a flight level (i.e. 35000ft) and when the flight guidance computer shows a higher optimal altitude, the pilots ask for clearance and eventually can climb to the next level (i.e. 37000ft).
Roughly 30min before the landing the descend is initiated (can vary obviously) and can be a step descend (level to level) or a continious descend (the most economical way to descend).

What is the flight trajectory of a commercial airplane ...

Circular Arc
A flight from say, Aukland to Doha would follow a path through three-dimensional space that is approximately circular. The great-circle distance along the track is 14,535 kilometres, the vertical variation above the roughly spherical surface track is about 10 km - less than 0.07% - we can ignore that.

Google Earth showing shortest route from Aukland to Doha in red. The thickness of the red line is about 22 times the cruising altitude of an airliner. The difference between ground level and 35000 feet is maybe half a pixel.
Any graphs of long-distance flights that don't look circular are heavily distorted statistical deceptions designed to show only the insignificant† tiny variations from a perfectly smooth curve.

...in regards to altitude v/s distance?

A Straight line
See above.

† From a geometric perspective. You pilots keep watching that altimeter!


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Are there rules regarding the accuracy of a pilot's delay announcement?
Recently in an advertisement, I saw that a pilot gets notified that the plane will be delayed by 30 mins, but he announces that the plane will be delayed by 3 hours. And then he makes another announcement with the update saying that the flight will now be delayed by only 30 mins with the passengers shown cheering. I can understand the human psychology behind it but I think that there must be some rules and regulations about such fake announcements. Can someone tell me about it?

Sounds like the capt has been burned by Maintenance Control one too many times.

Do note that this is a question about something that happened in a TV commercial, not an actual airline event.

Yup, I know it is not an actual event that has happened. But the point is, what if this stuff actually happened. If a guy sitting in an office can think this up, then maybe one day a pilot with a sense of humour and a fascination for human psychology decides to do this, are there any official rules or regulations about it or not?

Here is your answer https://www.youtube.com/watch?v=t9SVhg6ZENw

No precise rules that come to mind offhand, but:

these days, the passengers often get text messages with delay info, and if I'm saying "3 hours" as the text comes in saying "30 minutes", that is not good
people who might wait out a 30 minute delay might decide to rebook or cancel their flight for a 3 hour delay
if there are passengers making connections to other flights at the destination, the difference to them between 30 minutes delay and 3 hours is immense, and they can get upset, call the airline reservations people, want off the aircraft to talk to customer service, etc
an actual delay of 3 hours is reason to deplane everybody, now (in most cases); for 30 minutes, not so much. That process & subsequently re-boarding everyone takes time, and it's no fun for anybody.
play the "oh, wait, now the delay is xxxx" card too often, and you look like you don't know what you're doing.

My gameplan is to give the passengers the honest answer. Sometimes, this is along the lines of, "may be a quick fix, like 15 minutes, or it may be a lengthy delay, in the ballpark of 2-3 hours. We expect to know which situation we have in about 15 minutes or so, and we'll let you know when we get the word from Maintenance." I think people appreciate it when you give them the straight scoop, including the times when the crystal ball is cloudy.

Spent my life dealing with people and learned that honesty always wins, you don't have to be right just be honest

The question isn't whether it's a good idea to do what the advertisement suggests, it's whether it's legal. This answer doesn't really address that.

I agree with what you said, but from your answer, there don't seem to be any official regulations about it. But in your case itself, what if the flight is to a remote island where you don't expect to have connecting passengers from and only 1 flight in a day or two. What then?

@DanHulme  The first sentence exactly addresses the lack of any legal requirements, as far as I'm aware. I'm not sure how better to address or support such a situation than to say "none, AFAIK" and move on. Is there a better method to prove a negative?


text:
How to find flying directions (similar to driving directions) for navigation?
Similar to driving directions, is there a service from where we can get flying directions? Let's suppose an airplane does not have a GPS and it has to rely on VORs for navigation.
I know about SkyVector. But if I create a flight plan there, it is a straight line. They also specify that this is not for navigation.

Many GA airplanes rely on the pilot's eyes combined with dead reckoning for navigation and are perfectly capable of flying direct :)

I don't really know what to say.  Basic navigation with VORs and paper charts is taught to every student prior to the student taking the written and practical tests.  I guess this didn't stick in your case....

@SkipMiller Perhaps the individual asking the question is not yet a pilot (student or otherwise) or is asking a question that would be perfectly reasonable for a curious non-flying individual.

Terry, you are right.  Perhaps the best answer for an individual who has aspirations of becoming a pilot is to find a CFI or CFI-G and take a lesson or two.

I would like to note that I am one of those who hang around because I am interested in aviation, for education and entertainment, without a chance to become a pilot, ever. And hope it is not frowned upon :-)

@Peter Masiar:  Let me be the first to say no, it is not frowned upon.  There are lots of ways the OP could be asking a legitimate question and my first comment overlooked all that to assume the OP was a pilot.  My Bad!  OK carry on!  May I suggest the OP look at www.vfrmap.com as a first step...

The traditional (i.e., "old") method is to spread out aviation charts on a large table, and use a plotter to hand-craft a flight plan on paper. Victor airways (the VOR airway network) are depicted both on VFR sectionals and on IFR low & high altitude enroute charts. The pilot/navigator will mark his or her origin and destination, and eyeball the available airways to come up with a suitable route, much like a driver would use a national atlas to find what interstates/highways to take for a long road trip. These maps depict all the information needed about distances, courses, minimum altitudes and airspace classifications.
Planning by hand may also involve the use of the Airport/Facility Directory (A/FD), which has information about airports, and the Terminal Procedures Publication which shows what instrument approaches are available at each airport and any necessary IFR departure procedures, takeoff weather minimums, etc.
Nowadays, people will more often use computer planning tools like AOPA Flight Planner, fltplan.com, etc. There are a large number of sites and software programs for this, some free, some requiring paid subscription / purchase. Most recently, mobile apps for smartphones and tablets have emerged with the same capabilities.
The best such sites/software will also provide the necessary information about terrain, takeoff minimums, departure procedures and so on, and even give you current NOTAMs, weather forecasts, etc. Many also have aircraft performance databases (or allow you to create one yourself for your airplane) and can do performance calculations for you. Some will even find the cheapest places along your route to purchase fuel, and also give you information about local transportation and accomodations.
Note, though, that neither the FAA nor any other civil aviation authority authorizes the pilot to rely on software resources. The pilot is still always responsible for verifying that he or she has complete, accurate information from official sources. In other words, if you're using software to plan your flight, and you violate restricted airspace that your flight planning software didn't warn you about, you'll be held responsible for not verifying the information and catching the error. So be careful what you use and rely on!

The tools you link to seem to be US/Americas only. If anyone could expand the answer to include links to European or world-wide tools, it would be very useful.

@VidarS.Ramdal Unfortunately outside the US access to aviation data is "extremely variable" (which is a polite way of saying you're probably not going to find the equivalent level of access without forking over some money). If you can turn up something similar we'd all definitely be interested though :)

Yeah, I suspected that. Thanks, at least I know there's not much point in searching around for it.

A word of caution about the electronic tools: they are great and make things INCREDIBLY easy, but it's still important to have a good grasp on how to do it "the old way". I was teaching when GPS started coming online for civilian use, so the students loved to "Ooh" and "Ahh". We had one airplane that had a GPS in the panel, and one of my students wanted to use it for a cross country. He planned and plotted the flight, programmed it in, began flying the path, and I turned the GPS off. He asked "Why?" so I said "GPS failure". He scrambled with the "old way" charts to figure out our path.


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How can one calculate the maximum instantaneous bank angle a plane can do in a certain time?
I know that there is a limitation to do an instantaneous banking angle a plane can do. These limitations are due to the airplane (infrastructure - aerodynamics) and limitation on the pilot (due to his ability to withstand a certain maneuvering calculated in g), so how can one calculate an accurate or approximate value for the maximum instantaneous bank angle taking into account the (military and civilian) planes infrastructure and also taking into account the pilot (with or without) a g-suit.
Also i want to know how long can a pilot withstand that instantaneous bank angle?
Thanks in advance.

I assume you're asking about the maximum bank angle in a level turn; is that right? Outside of a level turn, many planes can easily bank through 360 degrees.

This question has been asked and answered in over 20 various forms, a few are....
How much g-force is experienced in a 45° turn?
What is the relation between airspeed and rate of turn?
Best procedure to Turn around in Canyon, Turn radius as a function of velocity
How to calculate angular velocity and radius of a turn?

you did not answer the last question "Also i want to know how long can a pilot withstand that instantaneous bank angle?" could you answer it ?

If by instantaneous you mean what is commonly called instationary, the rules of the turn allow to trade altitude for turn rate such that the  maximum load factor can be achieved even if the drag involved in doing so far exceeds the thrust available from the engines.
Contrast this with a stationary turn were no more drag may be created than can be handled by the engines such that altitude is maintained.
Then the answer is easy: The maximum bank angle $\varphi$ for a coordinated, instationary turn can be obtained from the maximum g load factor $n_z$ the plane can sustain. For small vertical speeds, use the formula for level flight:
$$\varphi = \arctan\left(\sqrt{n_z^2-1}\right)$$
If the vertical component of the flight path angle $\gamma$ should not be neglected, the load factor can become slightly higher since gravity has a forward component which does not add to the loads in the z-direction:
$$\varphi = \arctan \left (\sqrt{n_z^2-\cos\gamma}\right)$$
The answers to this question discuss the issue in more detail.
As to the time the pilot can sustain the bank angle: This is really about the load factor, and for that we have Eiband diagrams. Essentially, much depends on pilot position and the techniques used to avoid blackout. A few trained pilots when sitting can suffer through 9g for a few seconds, but a more normal limit would be 12g (equivalent to $\varphi = 85^\circ$) for 0.04 seconds, decreasing to 5 g (equivalent to $\varphi = 78^\circ$) when durations in excess of 0.2 seconds are involved.


text:
How do Class B transitions work in the US?
I'm just wondering about VFR requests relating to class bravo airspace in the US:

Is requesting VFR flight following in a class bravo the same thing as requesting a class bravo clearance?
Does requesting a class bravo airspace transition automatically include VFR flight following (radar traffic information service)?
If I am cleared into the bravo airspace and the controller gives me a heading and altitude to fly that clips the airspace of an underlying class C / D airspace, have I satisfied the requirements to enter said airspace (class C - 2-way communication and squawking mode C, class D - 2-way communication)?
Why does ATC provide traffic advisories for VFR aircraft in class bravo airspace at all if they are on the hook for providing separation?

Related: Does Flight Following automatically include permission to fly through class C or D airspace?

There are a lot of questions here, and you may get more in-depth answers if you split them up into different ones on the site!

Is requesting VFR flight following in a class bravo the same thing as requesting a class bravo clearance?

No. Flight following just means that the aircraft is receiving traffic advisories. A VFR aircraft needs explicit clearance to enter the Class B. The pilot should net ATC know that they intend to enter the Class B.

Does requesting a class bravo airspace transition automatically include VFR flight following (radar traffic information service)?

Yes, ATC must separate VFR aircraft from VFR/IFR aircraft while in the Class B. (7110.65X Section 7-9-4)

If I am cleared into the bravo airspace and the controller gives me a heading and altitude to fly that clips the airspace of an underlying class C / D airspace, have I satisfied the requirements to enter said airspace (class C - 2-way communication and squawking mode C, class D - 2-way communication)?

If ATC has given specific instructions that take a pilot through other airspace, then ATC should coordinate this with the appropriate controllers. Per 7110.65X Section 7-9-2 the controller should also either vector aircraft to stay in class B or to inform them if they are leaving it. Pilots should not change frequencies on their own unless asked, and the pilot should ask the controller if they are unsure.
Also, note that:

Assignment of radar headings, routes, or altitudes is based on the provision that a pilot operating in accordance with VFR is expected to advise ATC if compliance will cause violation of any part of the CFR.

Why does ATC provide traffic advisories for VFR aircraft in class bravo airspace at all if they are on the hook for providing separation?

As described above, ATC is required to separate VFR aircraft from other aircraft in the Class B. ATC is instructed by 7110.65X Section 2-1-21 to:

...issue traffic advisories to all aircraft (IFR or VFR) on your frequency when, in your judgment, their proximity may diminish to less than the applicable separation minima.

Unless a VFR aircraft has explicit instructions, ATC does not know for sure what they will do. And IFR aircraft may still need to perform unexpected maneuvers like a missed approach. Traffic callouts make it easier to apply visual separation if needed.

Thanks. If ATC tells aircraft to maintain visual separation, can they legally allow less than the prescribed radar separation (3 miles, or green between targets)? I was thinking that maybe they used visual separation so that they could increase the number of aircraft in the area but still remain safe and legal.

@jeff0000 Yes, the separation options here are horizontal, vertical, or visual. One of the three is sufficient.

If you request VFR flight following that will transition Class Bravo, you will either be explicitly cleared into the Bravo, or not - perhaps your path will take you under/through the arriving/departing active runway that day, and you will be denied Bravo clearance, and can request it again in a few miles, or they may clear you to a high altitude without asking once you will not be in the way.  I've had it go both ways flying in the Boston, MA area.
Yes, sort of - IFR traffic will be warned of you, you may not be told of IFR traffic near you.
Last time I was tracked to overfly an underlying airspace with flight following, the Class B originated flight following cleared me thru the underlying Class C, and I didn't talk to the Class C folks at all.  The flight following should stay with you until you clear the various class boundaries - or may stay with you to your destination. Really depends where you are flying and if you remain high enough for them to 'see' you.  For example, I flew from west of Boston all the way out to Nantucket Island - was handed off between several controllers on the way, including the Class C in Providence, RI, for a few of the miles.
Seperation is for IFR, with VFR on time permitting basis. See #2.

Thanks. By the way, I found this FAA document that says that all aircraft (IFR and VFR) are separated in class bravo airspace: https://www.faasafety.gov/gslac/ALC/course_content.aspx?cID=42&sID=505&preview=true "An ATC clearance is required for all aircraft to operate in Class B Airspace, and all aircraft that so cleared receive separation services within the airspace. The cloud clearance requirement for VFR operations is "clear of clouds"."

Even better!  Good to know.

VFR flight following will NOT necessarily result in an explicit statement that you are to remain clear of the Bravo. A helpful controller might prompt you ("Skykisser A-BCDE, you are approaching the Bravo; say intentions"), but it is the PIC's responsibility to request the clearance, and to remain outside unless explicitly cleared in.

FYI, there are not separate "class B folks" and "class C folks". Despite the different chart labels, the same TRACON facility will handle all adjacent B/C (except what Tower handles near the surface) plus the E above/below/around, and their sectors often don't match class lines.


text:
What is the correct name for clearance delivery at KSAN if the tower is called "Lindbergh Tower"?
In the departure and instrument approach charts for KSAN, they reference Lindbergh Tower" for KSAN tower. Does this mean that we should be calling up "Lindbergh Clearance" to pick up our flight plan?

http://155.178.201.160/d-tpp/1807/00373IYLY9.PDF  Says on the chart it's SOCAL Approach Control

I would say it‘s „Lindbergh Delivery“, as usual

@CrossRoads that is an approach frequency listed on an approach plate.  Nothing to do with CD.

Lindbergh is used as an identifier shortcut because "Lindbergh Field" is the official name of the airport. So I would call in with "Lindbergh Clearance Delivery".


text:
What does it mean when ATC says "altitude indicates"?
I've noticed that ATC sometimes calls out traffic (a traffic advisory?) to aircraft using the "altitude indicates" phraseology, and sometimes a specific altitude is given without the phrase "altitude indicates".
Is there a specific reason for when they use one over the other?

"Altitude indicates" means that the aircraft has a Mode C transponder but it isn't receiving radar service, i.e. it's VFR and not on flight following. In other words, the transponder is reporting an altitude, but because the aircraft isn't in contact with ATC, they can't be sure about its altimeter's integrity or the pilot's intentions.
See the ATC orders 2-1-21:

For aircraft displaying Mode C, not radar
  identified, issue indicated altitude

Mode C is unaffected by altimeter setting

To follow on @user71659 - transponders report the pressure data directly before converting to an altitude.  Since ATC always knows the most current atmospheric pressure it is always more accurate for them to calculate the altitude directly rather than to rely on whatever setting the pilot may have selected in the cockpit.  This guarantees a single conversion factor and ensures their display gets the relative position of all aircraft correct.

When you first check in with ATC, you tell them your altitude. If it doesn’t match what they are seeing, they will ask you to verify. If it is far off, they sometimes ask you to turn off your altitude squawk.

There's 2 types of radar in ATC, there's Primary Radar which gives bearing and distance, but it can't give altitude, which is why there is Secondary Surveillance Radar (SSR) which interrogates airplane transponders. When a transponder is on and set to mode C or S the controller will see flight level on their screen. Flight level is altitude adjusted to standard pressure (1013hPa):

When a controller gives a traffic report and they haven't had a reported altitude from the aircraft they'll either report the aircraft's transponder reported flight level or altitude in feet which is the transponder reported flight level adjusted for local pressure. A controller may also use the transponder reading if the airplane has changed altitude since its last report, say because they are maneuvering, doing practice forced landings, etc.

MSL is wrong here no? Mode C will be flight level I thought?

Good point @jk, I have edited to clarify.

When below transition altitude, ATC will see AMSL altitude, not flight level. The transponder transmits the flight level but the automation translates this into an altitude based on the local QNH.

ATC is looking at the altitude reported by the aircraft's mode C transponder. They have to talk to that aircraft and verify that the altitude is correct before they can trust that information. This is part of "radar identifying" a target. If a target is not radar identified, they just say "altitude indicates."

"Altitude indicates" means it is a random target and the controller is going by the only information available, the Mode C encoded altitude readout showing on the display.  If that phrase is not used, the controller is likely referring to an aircraft flying an altitude associated with a clearance as confirmed by the pilot (and backed up by the encoded altitude displayed for that target also).


text:
Official way for tower to address all aircraft on frequency for new ATIS?
When there's new ATIS info available and the altimiter setting has changed, what's the official phraseology for making the announcement to all the aircraft on the frequency? I'm looking in the FAA ATC phraseology doc FAA order JO 7110.65W, but I don't see it.
My best guess would be something like "Phoenix traffic, information sierra is current, Phoenix altimiter two niner niner eight".

What makes you think there needs to be specific phraseology for this in the first place, rather than simply the new information superceding the old? (If Sierra is currently being broadcast and a pilot reports having information Romeo, they can be corrected.) If the pilot repeatedly misses the automated ATIS broadcast, what are the odds that they'll hear a single transmission whether manual or automated? I dare say most radio-equipped aircraft have some form of dual-receive capability, whether via dual watch on a single radio or by having two radios for voice communications.

@user, because most people practicing touch and goes in the pattern aren't going to tune ATIS on the downwind.

The tower will usually broadcast a note such as “Attention all aircraft: ATIS information Victor is now current. Wind two two zero at seven knots.  Altimeter three zero one four”.  This isn’t official phraseology and really isn’t required of ATC, but is a courtesy call to all pilots operating around the airport to retrieve the latest ATIS broadcast for reference.

This answer is more-than-partially incorrect. ATC is required to broadcast an advisory when the ATIS changes, as I explain in my answer.

You're looking for JO 7110.65 paragraph 2–9–2d. There is no "official" PHRASEOLOGY defined there, but it does specify that controllers need to make a general broadcast which includes an update to pertinent information and provides some examples.

d. Advise aircraft of changes to the ATIS code by broadcasting the change on all appropriate frequencies. The broadcast must include changes to pertinent operational information, when known, that necessitated the ATIS change.
EXAMPLE-
“Attention all aircraft, information ALPHA current.”
“Attention all aircraft, information BRAVO current. MICROBURST advisories in effect.”
“Attention all aircraft, information CHARLIE current. Numerous flocks of ducks in the immediate vicinity of (name) airport, altitude unknown.”
NOTE-

No additional acknowledgement is required when a controller broadcasts information subsequent to the pilot's initial acknowledgement of the ATIS. [snip]
Pertinent conditions are those that have a clear decisive relevance to the safety of air traffic. As noted in paragraph 2-1-2, Duty Priority, there are many variables involved that make it virtually impossible to develop a standard list... [etc, etc, use best judgment].

My technique when broadcasting a "new ATIS" message due to an updated hourly observation, but no other change, is to include the wind and altimeter if I know there is an aircraft on frequency who might not have time to go listen to the new ATIS.

And we appreciate you doing that!

There have been many occasions when we were monitoring Tower, or Approach, both before and after contacting either, and heard a one time a announcement that the ATIS had been updated, with phraseology as you indicate. I don't recall if the altimeter was included.  Altimeter is given out pretty liberally to anyone contacting Approach/Tower, so you can pick that up if you can determine you are in the same area (have to pay attention, particularly for approach, as they cover a wider area).

The Navy uses the call sign 99 when the transmission applies to all.  “99 information charlie is current.”  I realize this doesn’t apply in the general sense, but it is an example of what you were asking about.


text:
What is different between a B737 and B757?
Both are single aisle, 3 by 3 seating planes.  Both are vaguely similar in length (with appropriate models).  But they have fairly different ranges.
For similar sized airplanes, why the big difference?

check the specs on wikipedia: B737 and B757

the 757's engines are far more powerful than the 737's.

Although the 737 has grown over the years, the 737 and 757 remain in different size classes. Both models can be ETOPS certified, although the 757 is more often used in long flights.
At the time the 757 was introduced (in the 80's), the 737 classic series was current. For the 737-400:
Passengers: 146
Length: 119 feet
Wingspan: 95 feet
Wing area: 1135 sq feet
MTOW: 138,000 lb
Takeoff distance: 8000 feet
Range: 2160 nm
Thrust: 2x 23,500 lbf
Max fuel: 5,311 US gal
The 757 was significantly larger. For the 757-200:
Passengers: 200
Length: 155 feet
Wingspan: 125 feet
Wing area: 1951 sq feet
MTOW: 255,000 lb
Takeoff distance: 6500 feet
Range: 4100 nm
Thrust: 2x 43,500 lbf
Max fuel: 11,489 US gal
The 757 is about 35 feet longer, seats about 50 more passengers, and flies about 2,000 nm further (roughly twice as far). The extra range is due to the larger wingspan and longer fuselage, being able to carry more fuel. For this reason, the 757 is used on longer and busier routes, while the 737 is used on shorter, less busy routes.
In the 90's, both models were updated. The 737 was significantly upgraded, including new wings and a new engine, which increased its efficiency. The 737 NG series eventually included the 737-900ER model, currently the largest 737 model.
Passengers: 180
Length: 138 feet
Wingspan: 117 feet
Wing area: 1341 sq feet
MTOW: 187,700 lb
Takeoff distance: 9843 feet
Range: 3235 nm
Thrust: 2x 28,400 lbf
Max fuel: 7,837 US gal
The 757-300 was also introduced. This was a stretched version of the 757-200.
Passengers: 243
Length: 178 feet
Wingspan: 125 feet
Wing area: 1951 sq feet
MTOW: 272,500 lb
Takeoff distance: 7800 feet
Range: 3595 nm
Thrust: 2x 43,500 lb
Max fuel: 11,489 US gal
Although the more efficiency gains of the new 737 bring the range up to nearly that of the 757, the 757 remains significantly larger.

There's a good point implicit in here: it's important not to compare the largest 737 with the smallest 757 but to compare the models that were available at a particular time.

Of particular importance here: compare the MTOW of the planes. While the 757-200 only carried 33% more passengers than the 737-400, its MTOW was 85% higher, allowing it to carry more than twice the fuel of the 737-400.

Of interesting note, the newest/largest 737 is still smaller (in all measures listed) than the original 757.

Aaand the largest 737 is now the MAX 10, which has a higher maximum passenger capacity than the 757-200 (230 vs. 200) and is, physically, only marginally smaller.

The 757 was designed after the oil shock of 1973 and represented the best which was possible in fuel efficiency. Therefore, its range is much greater, and with ETOPS certification it can be used on longer overwater trips. Besides, it shares the cockpit layout of it's twin aisle sibling 767, so flight crews could be certified on both with a single training. It replaced the Boeing 727, which is a bigger airplane than the 737.
The 737 is a much older design, but has since then been redesigned twice. The latest models (737-600 to 737-900) were redesigned in the 1990ies and have a new wing and new engines, so their fuel efficiency is again state of the art. Both the 737 and the 757 inherited their fuselage cross section from the 707. With the stretched fuselage of the 737-900, the latest 737 it is almost as big as a 757-200.

Could you elaborate on how the 737's cross section was inherited from the 707? Otherwise, nice answer and +1 :)

@shortstheory The 707 and 737 share the same nose for instance.

Thanks, MikeFoxtrot! @shortstheory, please see this link. The 727 also inherited the 707 fuselage, which had a constant cross section for most of its length, so the designers simply cut out sections. They could use doors, galleys, the interior paneling and many fittings from the 707. The commonality is approx. 60%, which helped to get the plane from project launch to first flight in just two years.

I just learned from this thread at airliners.net that the forward fuselage cross section of the 757 is smaller than the aft section. Interesting stuff! The forward section was taken from the 727, and the aft section from the 737. The upper part was the same for all three; the difference was in the lower part.